Is it possible to verify the password or decrypt of a the password of a PST file?

There is no API which allows the specification of a PST password for verification or logging-in. The password in the MapiLogonEx function exists for back-wards compatibility in with existing mapi code. The password was used when accessing older email system (MS Mail, etc) and is not used when accessing Outlook. The PST is loaded by Outlook one time and new MAPI sessions which are created will reference this one PST instance - even if there are two MAPI sessions. So, there is no existing supported method of verifying PST passwords. Also, there is no supported method for decrypting the password for a PST.